FIG. 1 is a circuit configuration diagram of a conventional DC-DC converter described in Japanese Patent Application Publication No. 2006-262601. This step-up DC-DC converter includes a DC power supply Vdc1, transformers T3 and T4, a reactor L3, switches Q1 and Q2, diodes D3 and D4, a smoothing capacitor C1 and a control circuit 100.
The transformer T3 has a primary winding 5a (np turns), an additional winding 5b (np1 turns) connected to the primary winding 5a in series, and a secondary winding 5c (ns turns) electromagnetically coupled to the primary winding 5a and the additional winding 5b. The transformer T4 is configured in the same way as the transformer T3, hence having a primary winding 6a (np turns), an additional winding 6b (np1 turns) connected to the primary winding 6a in series, and a secondary winding 6c (ns turns) electromagnetically coupled to the primary winding 6a and the additional winding 6b. 
The drain and source of the switch Q1 formed of a MOSFET or the like are connected to both ends of the DC power supply Vdc1 through the primary winding 5a of the transformer T3. The drain and source of the switch Q2 formed of a MOSFET or the like are connected to both ends of the DC power supply Vdc1 through the primary winding 6a of the transformer T4. A first series circuit, which includes the additional winding 5b of the transformer T3, the diode D3 and the smoothing capacitor C1, is connected to a node at which the primary winding 5a of the transformer T3 and the drain of the switch Q1 are connected, and to the source of the switch Q1. A second series circuit, which includes the additional winding 6b of the transformer T4, the diode D4 and the smoothing capacitor C1, is connected to a node at which the primary winding 6a of the transformer T4 and the drain of the switch Q2 are connected, and to the source of the switch Q2.
The reactor L3 is connected to both ends of a series circuit formed of the secondary winding 5c of the transformer T3 and the secondary winding 6c of the transformer T4. The control circuit 100 turns on and off the switches Q1 and Q2 based on an output voltage Vo of the smoothing capacitor C1 with a phase difference of 180°.
In the conventional DC-DC converter configured as above, the switch Q1 is turned on by a Q1 control signal Q1g from the control circuit 100. Then, an electric current flows through a path from the positive side of Vdc1 to the negative side of Vdc1 through 5a and Q1. Accordingly, an electric current Q1i in the switch Q1 linearly increases. At the same time, a voltage is generated also across the secondary winding 5c of the transformer T3, and an electric current L3i flows through the reactor L3 by flowing through a path from 5c to 5c through L3 and 6c. 
This electric current L3i flows in accordance with the law of equal ampere-turns of transformers, causing energy to be stored in the reactor L3. At the same time, the same electric current flows through the secondary winding 6c of the transformer T4. Accordingly, across the primary winding 6a of the transformer T4 and across the additional winding 6b thereof, voltages corresponding to the respective numbers of turns are induced.
When an additional winding ratio of the transformer T4 is A=(np+np1)/np, an electric current, which is 1/A of the electric current Q1i in the switch Q1, flows through the diode D4 by flowing through a path from the positive side of Vdc1 to the negative side of Vdc1 through 6a, 6b, D4, and C1. The electric current D4i in the diode D4 flows until a time at which the switch Q2 is turned on. The output voltage Vo across the smoothing capacitor C1 is the sum of a voltage across the DC power supply Vdc1 (an input voltage), a voltage generated across the primary winding 6a of the transformer T4, and a voltage generated across the additional winding 6b of the transformer T4.
When a duty factor of the switch Q1 is D (D=Ton/T), the voltage generated across the transformer T4 is A·Vdc1·D, where Ton is a period of time during which the switch Q1 is on, and T is a cycle in which the switch Q1 is switched. The output voltage Vo across the smoothing capacitor C1 is Vo=Vdc1(1+A·D). Accordingly, the output voltage Vo can be controlled by changing the duty factor D.
Next, the switch Q1 is turned off by the Q1 control signal Q1g from the control circuit 100. Then, an electric current D3i flows through a path from the positive side of Vdc1 to the negative side of Vdc1 through 5a, 5b, D3, and C1.
Next, the switch Q2 is turned on by the Q2 control signal Q2g from the control circuit 100. Then, an electric current D3i flows through a path from the positive side of Vdc1 to the negative side of Vdc1 through 6a and Q2. Accordingly, an electric current Q2i in the switch Q2 linearly increases. At the same time, a voltage is generated also across the secondary winding 6c of the transformer T4, and the electric current L3i flows through the reactor L3 by flowing through a path from 6c to 6c through 5c and L3 while increasing.
This electric current L3i flows in accordance with the law of equal ampere-turns of transformers, causing energy to be stored in the reactor L3. At the same time, the same electric current flows through the secondary winding 5c of the transformer T3. Accordingly, across the primary winding 5a of the transformer T3 and across the additional winding 5b thereof, voltages corresponding to the respective numbers of turns are induced.
When an additional winding ratio of the transformer T3 is A=(np+np1)/np, an electric current, which is 1/A of the electric current Q2i in the switch Q2, flows through the diode D3 by flowing through a path from the positive side of Vdc1 to the negative side of Vdc1 through 5a, 5b, D3, and C1. The electric current D3i in the diode D3 flows until a time at which the switch Q1 is turned on. The output voltage Vo across the smoothing capacitor C1 is the sum of a voltage across the DC power supply Vdc1 (an input voltage), a voltage generated across the primary winding 5a of the transformer T3, and a voltage generated across the additional winding 5b of the transformer T3.
As described above, in the multiphase, transformer-linked, step-up chopper circuit shown in FIG. 1, two phases, which are independent of each other, are coupled by transformers. This allows boosting operation using only one core, instead of two or more cores needed without the coupling.